1. Field of the Invention
The present invention relates to a method of producing ceramic laminates, laminated electronic parts and a method of producing the parts. In particular, the invention relates to a method of producing ceramic laminates in which ceramic green sheets and electrically conducting patterns are laminated in many layers having small thicknesses like in a wiring board or a laminated ceramic capacitor, to electronic parts of a type in which many thin ceramic layers are laminated, and to a method of producing the same.
2. Description of the Related Art
Accompanying the trend toward realizing electronic devices in ever small sizes and in high densities in recent years, it has been urged to provide laminated electronic parts in small sizes having reduced thicknesses yet maintaining a high dimensional precision so as to be mounted in the electronic devices. In the field of wiring boards having electrically conducting patterns formed in the ceramic laminate and electronic parts such as laminated ceramic capacitors, efforts have been made to decrease the thickness of the electrically conducting patterns and to employ a multi-layered structure constituted by ceramic green sheets and inner electrode layers that serve as the dielectric ceramic layers from the standpoint of decreasing the weight, decreasing the thickness, decreasing the size and increasing the capacity.
In the ceramic laminate for the electronic parts, the thickness of the electrically conducting patterns formed on the ceramic green sheet exhibits an increasing effect as the thickness of the ceramic green sheets decreases and as the ceramic green sheets are laminated in an increased number one upon the other. Due to the thickness of the electrically conducting patterns, therefore, a difference in level builds up between the portions where the electrically conducting patterns are formed and the portions where they are not formed, resulting in a decrease in the adhesion among the ceramic green sheets in the portions where there is no electrically conducting pattern, and developing delamination and cracks.
Therefore, a contrivance has been made to eliminate a difference in level on the ceramic green sheet.
A method of producing such a ceramic laminate for electronic parts has been disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 311831/2000.
In the method of producing the ceramic laminates disclosed in this publication as shown in FIG. 9, ends 85 of electrically conducting patterns 83 are so formed as to possess inclined surfaces 87 defining an acute angle with respect to the main surface of a ceramic green sheet 81 in a step of forming the electrically conducting patterns 83 on the main surface of the ceramic green sheet 81, and a ceramic paste is applied so as to be overlapped on the inclined surfaces 87 of the electrically conducting patterns 83 in a step of applying the ceramic paste to the peripheries of the electrically conducting patterns 83.
According to the above production method, the inclined surface 87 are formed at the ends 85 of the electrically conducting patterns 83. Therefore, the ceramic paste applied so as to be overlapped on the inclined surfaces 87 quickly migrate into among the electrically conducting patterns 83 and is smoothly leveled without substantially creating a difference in level despite of the thickness of the electrically conducting patterns 83. Accordingly, the ceramic green sheets 81 can be laminated without being affected by the thickness of the electrically conducting patterns 83.
To decrease the cost of electronic parts in recent years, however, a large number of ceramic laminates are taken out from a mother laminate. For this purpose, the ceramic green sheet 81 and the printing screen are now being produced having large work sizes. For example, there has been used a printing screen in which semiconductor patterns each having an area of about not larger than 1×2 mm2 are arranged maintaining a gap of not larger than about 0.5 mm, the effective size of the printing screen being not smaller than 150×150 mm2.
When the ceramic paste is printed by using the printing screen having such a large effective size, the elongation factor thereof becomes large toward the peripheral portions of the printing screen as compared to the central portion due to the pressure of printing, resulting in an increase in the positional deviation of the ceramic patterns 89 formed among the electrically conducting patterns that have been formed in advance on the ceramic green sheet 81 and, particularly, among the electrically conducting patterns 83 formed in the peripheral portions.
That is, the printing screen has a structure in which the outer periphery of the screen is secured to a frame of a rectangular shape, and effects the printing by moving a blade from one end of the screen toward the other end thereof while being pushed onto the screen. As the blade is pushed onto the screen and is moved, however, the elongation of the screen becomes great toward the peripheral portions as compared to the central portion, and the deviation in the position of printing the ceramic pattern 89 increases toward the peripheral portions.
In the method of producing the ceramic laminate for electronic parts disclosed in the above Japanese Unexamined Patent Publication (Kokai) No. 311831/2000, the ceramic paste is applied so as to be overlapped on the inclined surfaces 87 of the electrically conducting patterns 83, and the ceramic paste applied onto the ends 85 of the electrically conducting patterns 83 migrate into among the electrically conducting patterns 83 and is leveled. As described above, the ceramic paste printed by using the peripheral portions of the printing screen is greatly deviated in position. Even if the position of the printing screen is so controlled that the ceramic paste is just applied onto, or applied slightly riding on, the ends 85 of the electrically conducting paste prior to effecting the printing, the ceramic paste is not just or suitably overlapped on the inclined surfaces at the ends of the electrically conducting paste in the peripheral portions of the printing screen, arousing a problem in that the ceramic paste is applied greatly riding on the ends 85 of the electrically conducting patterns 83.
Accordingly, the ceramic pattern 89 swells on the flat portions 91 near the ends 85 of the electrically conducting patterns 83. The thickness locally increases in the portions printed by using the peripheral portions of the printing screen, and the ceramic laminate develops delamination and cracks.
That is, according to Japanese Unexamined Patent Publication (Kokai) No. 311831/2000, the ceramic paste is applied onto the inclined surfaces 87 of the electrically conducting paste. Therefore, if the position of the printing screen is strictly controlled, the ceramic paste can be applied to be just or suitably overlapped onto the inclined surfaces 87 of the electrically conducting patterns in the central portion of the printing screen. In the peripheral portions, however, the ceramic paste is printed onto the flat portions 91 of the electrically conducting patterns 83. In the portions where the ceramic paste is applied onto the ends 85 of the electrically conducting patterns 83, the solvent contained in the ceramic paste infiltrates into the electrically conducting patterns 83 which are porous after the printing has been finished. Therefore, the ceramic paste is partly deposited on the flat portions 91 near the ends 85 of the electrically conducting patterns 83 causing the thickness to increase in these portions.
When the ceramic green sheets 81 on which the electrically conducting patterns 83 and the ceramic patterns 89 are formed as described above, are laminated in a number of, for example, not smaller than 100 layers and, particularly, not smaller than 200 layers, the thickness increases at the ends 85 of the electrically conducting patterns 83. Accordingly, the adhesion decreases among the ceramic green sheets at portions printed in the peripheries of the printing screen, developing cracks and delamination after firing, and decreasing the yield.
In the laminate of electronic part (laminated ceramic capacitor) disclosed in the above Japanese Unexamined Patent Publication (Kokai) No. 311831/2000, further, the dielectric patterns formed for eliminating a difference in level caused by the inner electrode layers are formed of a material having a composition same as the dielectric green sheet. Therefore, the coefficient of contraction due to firing of the dielectric patterns in the direction of thickness is smaller than the coefficient of contraction due to firing of the inner electrode layers in the direction of thickness, whereby the junction strength becomes weak on the interface between the dielectric ceramic layers and the inner electrode layers, developing delamination in the peripheries of the inner electrode layers.
Further, since the junction strength is weak on the interface between the dielectric ceramic layers and the inner electrode layers, cracks occur in the peripheries and at the ends of the inner electrode layers of the laminated electronic part at the time of soldering or thermal shock testing.
The delamination and cracks occur easily as the thickness of the dielectric ceramic layers decreases due to a difference in the contraction by firing between the dielectric ceramic layers and the inner electrode layers, and occur more easily when the thickness of the dielectric ceramic layers is not larger than 3 μm.